1. Field of the Invention
The present invention generally relates to a discharge tube, and more particularly to a discharge tube in which an upper discharge electrode and a lower discharge electrode are opposed to each other in an airtight cylinder, and an electric discharge is repeatedly generated between the discharging surfaces of the upper and lower discharge electrodes.
2. Description of the Related Art
For example, a HID (high intensity discharge) headlamp for automotive vehicle requires an ignitor circuit which generates a high-voltage trigger in order to turn on the light. The ignitor circuit is mainly comprised of a capacitor which charges the electricity, a transformer which generates the high-voltage trigger, and a switching discharge tube which generates a stable voltage pulse. In the following description, this switching discharge tube will be called the discharge tube.
As disclosed in Japanese Laid-Open Patent Application No. 10-335042, the above-mentioned discharge tube is comprised of an airtight cylinder made of an insulating material, such as ceramics, and first and second discharge electrodes arranged to the end openings of the airtight cylinder. A discharging gap is formed between the first discharge electrode and the second discharge electrode within the airtight cylinder, and the filler gas is enclosed in the airtight cylinder in an airtight manner.
In the above-mentioned discharge tube, an electric discharge is generated at the discharging gap of the airtight cylinder with the presence of the filler gas therein. Conventionally, the filler gas used is a mixture of argon (Ar) gas as the major component and hydrogen (H2) gas in a volume concentration above 0.5% and below 20%.
The development of the conventional discharge tube has been carried out with emphasis given on the generation of a stable voltage pulse. However, with the recent demand of high-density assembly of the ignitor circuit in the automotive HID headlamp, it becomes necessary to increase the output voltage of the secondary coil of the transformer in addition to the generation of a stable voltage pulse by the discharge tube.
FIG. 1A and FIG. 1B are diagrams for explaining a transition of the operating voltage of a conventional discharge tube immediately after a start of discharging.
The filler gas of the discharge tube of FIG. 1A and FIG. 1B is composed of 90% by volume of argon (Ar) gas and 10% by volume of hydrogen (H2) gas. In the following, all the chemical composition (%) of the filler gas is expressed in the volume concentration (percent by volume) unless otherwise specified.
Moreover, FIG. 2A and FIG. 2B are diagrams for explaining the results of measurement of an output voltage of the secondary coil of the transformer of the ignitor circuit to which the conventional discharge tube (the composition of the filler gas: 90% Ar+10% H2) of FIG. 1A and FIG. 1B is applied.
In addition, in the cases of FIG. 1A and FIG. 1B, the connection of the discharge tube is made in the “plus” direction and the “minus” direction, respectively. Namely, the direction of the connection is reversed between the cases of FIG. 1A and FIG. 1B.
As is apparent from FIG. 1A and FIG. 1B, after a start of discharging of the conventional discharge tube, the operating voltage is not reduced to the ground in a straight manner, but the phenomenon takes place in which the discharge voltage is raised for a certain period after the start of discharging, as indicated by the arrow A in FIG. 1A and FIG. 1B. In the following, this phenomenon will be called the rebound phenomenon and the discharge voltage at this time will be called the rebound voltage. The rebound phenomenon takes place regardless of whether the connection direction of the discharge tube is the “plus” direction or the “minus” direction as shown in FIG. 1A and FIG. 1B.
Moreover, as shown in FIG. 2A and FIG. 2B, the actual output voltage of the secondary coil of the transformer of the ignitor circuit at this time declines greatly, although the desired value of the output voltage is about 11 kV. This is because the decline of the output voltage is caused by the above mentioned rebound phenomenon.